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Earthquake
An earthquake (also known as tremors and temblors) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity. In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above this. s, 1963–1998]] Naturally occurring earthquakes Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior. Earthquake fault types There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. Earthquakes away from plate boundaries Where plate boundaries occur within continental lithosphere, deformation is spread out a over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms. Talebian, M. Jackson, J. 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran. Geophysical Journal International, 156, pages 506-526 All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes. Shallow-focus and deep-focus earthquakes The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers). These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure. Earthquakes and volcanic activity Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helens eruption of 1980. Earthquake clusters Most earthquakes form part of a sequence, related to each other in terms of location and time. Aftershocks An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock. Earthquake swarms Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park. Earthquake storms Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East. Size and frequency of occurrence Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia. Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years. Seismicity and earthquake hazard in the UK This is an example of the Gutenberg-Richter law. The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable. In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS. Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains. Humans can cause earthquakes for example by constructing large dams and buildings, drilling and injecting liquid into wells, and by coal mining and oil drilling. With the rapid growth of mega-cities such as Mexico City, Tokyo or Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.Global urban seismic riskEarthquake safety in Iran and other developing countries Effects/impacts of earthquakes in ruins and in flames after the 1755 Lisbon earthquake. A tsunami overwhelms the ships in the harbor.]] There are many effects of earthquakes including, but not limited to the following: Shaking and ground rupture Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings or other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation.On Shaky Ground, Association of Bay Area Governments, San Francisco, reports 1995,1998 (updated 2003) The ground-shaking is measured by ground acceleration. Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits. Ground rupture is a visible breaking and displacement of the earth's surface along the trace of the fault, which may be of the order of few metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.Guidelines for evaluating the hazard of surface fault rupture, California Geological Survey Landslides and avalanches Landslides are a major geologic hazard because they can happen at any place in the world, much like earthquakes. Severe storms, earthquakes, volcanic activity, coastal wave attack, and wildfires can all produce slope instability. Landslide danger may be possible even though emergency personnel are attempting rescue. Fires ]] Following an earthquake, fires can be generated by break of the electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, the deaths in the 1906 San Francisco earthquake were caused more by the fires than by the earthquake itself. Soil liquefaction Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, as buildings or bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, many buildings were sunk into the ground by soil liquefaction, eventually collapsing upon themselves. Tsunami ]] Tsunamis are long-wavelength, long-period sea waves produced by an sudden or abrupt movement of large volumes of water. In the open ocean, the distance between wave crests can surpass 100 kilometers, and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour, depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them. Ordinarily, subduction earthquakes under magnitude 7.5 on the richter scale do not cause tsunamis. However, there have been recorded instances, yet most destructive tsunamis are caused by magnitude 7.5 plus earthquakes. Tsunamis are distinct from tidal waves, because in a tsunami, water flows straight instead of in a circle like the typical wave. Earthquake-triggered landslides into the sea can also cause tsunamis. Floods A flood is an overflow of any amount of water that reaches land.MSN Encarta Dictionary. Flood. Retrieved on 2006-12-28. Floods usually occur because of the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which then collapse and cause floods. The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people. Human impacts Earthquakes may result in disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse of buildings or destabilization of the base of buildings which may lead to collapse in future earthquakes. Earthquakes can also lead to volcanic eruptions, which cause further damages such as substantial crop damage, like in the "Year Without a Summer" (1816). Most of civilization agrees that human death is the most significant human impact of earthquakes. Preparation for earthquakes Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: Earthquake engineering, Earthquake preparedness, Household seismic safety, Seismic retrofit (including special fasteners, materials, and techniques), Seismic hazard, Mitigation of seismic motion, and Earthquake prediction. Earthquakes in culture Mythology and religion In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr, god of beauty and light, he was punished by being bound in a cave with a poisonous serpent placed above his head dripping venom. Loki's wife Sigyn stood by him with a bowl to catch the poison, but whenever she had to empty the bowl the poison would drip on Loki's face, forcing him to jerk his head away and thrash against his bonds, causing the earth to tremble. In Greek mythology, Poseidon was the god of and cause earthquakes. When he was in a bad mood, he would strike the ground with a trident, causing this and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge. Popular culture In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906. Fictional earthquakes tend to strike suddenly and without warning. For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1998). The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977) among other works. See also *Earthquake insurance *Earthquake loss *List of earthquakes *List of all deadly earthquakes since 1900 *List of earthquakes by death toll References External links Educational * * How to survive an earthquake - Guide for children and youth *Guide to earthquakes and plate tectonics *[http://pubs.usgs.gov/gip/earthq1/ Earthquakes] — an educational booklet by Kaye M. Shedlock & Louis C. Pakiser *The Severity of an Earthquake *USGS Earthquake FAQs *IRIS Seismic Monitor - maps all earthquakes in the past five years. *Latest Earthquakes in the World - maps all earthquakes in the past week. *Earthquake Information from the Deep Ocean Exploration Institute, Woods Hole Oceanographic Institution *Geo.Mtu.Edu — How to locate an earthquake's epicenter *Photos/images of historic earthquakes *earthquakecountry.info Answers to FAQs about Earthquakes and Earthquake Preparedness *Interactive guide: Earthquakes - an educational presentation by Guardian Unlimited *Geowall — an educational 3D presentation system for looking at and understanding earthquake data *Virtual Earthquake - educational site explaining how epicenters are located and magnitude is determined *HowStuffWorks — How Earthquakes Work *CBC Digital Archives — Canada's Earthquakes and Tsunamis *Earthquakes Educational Resources - dmoz Seismological data centers Europe *International Seismological Centre (ISC) *European-Mediterranean Seismological Centre (EMSC) *Global Seismic Monitor at GFZ Potsdam *Global Earthquake Report – chart *Earthquakes in Iceland during the last 48 hours *Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy *Database of Individual Seismogenic Sources (DISS), Central Mediterranean *Portuguese Meteorological Institute (Seismic activity during the last month) Japan *Earthquake Information of Japan, Japan Meteorological Agency *International Institute of Seismology and Earthquake Engineering (IISEE) *Building Research Institute *Database for the damage of world earthquake, ancient period (3000 BC) to year of 2006- Building Research Institute (Japan) (建築研究所) in Japanese *Seismic activity in last 7 days - Weathernews Inc., indicated with circled shindo (震度） scale and it's location. **Weathernews Inc, Global web site New Zealand *GeoNet - New Zealand Earthquake Report (latest and recent quakes) United States *The U.S. National Earthquake Information Center *Southern California Earthquake Data Center *The Southern California Earthquake Center (SCEC) *Putting Down Roots in Earthquake Country An Earthquake Science and Preparedness Handbook produced by SCEC *Recent earthquakes in California and Nevada *Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer *Incorporated Research Institutions for Seismology (IRIS), earthquake database and software *IRIS Seismic Monitor - world map of recent earthquakes *SeismoArchives - seismogram archives of significant earthquakes of the world Seismic scales *The European Macroseismic Scale Scientific information * * Miscellaneous *Reports on China Sichuan earthquake 12/05/2008 *Kashmir Relief & Development Foundation (KRDF) *PBS NewsHour - Predicting Earthquakes *USGS – Largest earthquakes in the world since 1900 *The Destruction of Earthquakes - a list of the worst earthquakes ever recorded *Los Angeles Earthquakes plotted on a Google map *the EM-DAT International Disaster Database *Earthquake Newspaper Articles Archive *Earth-quake.org *PetQuake.org- official PETSAAF system which relies on strange or atypical animal behavior to predict earthquakes.(Link broken 03:33, 2 June 2008 (UTC)) *A series of earthquakes in southern Italy - 23 November 1980, Gesualdo *Recent Quakes WorldWide *Real-time earthquakes on Google Map, Australia and rest of the world *Earthquake Information - detailed statistics and integrated with Google Maps and Google Earth *Kharita - INGV portal for Digital Cartography - Last earthquakes recorded by INGV Italian Network (with Google Maps) *Kharita - INGV portal for Digital Cartography - Italian Seismicity by region 1981-2006 (with Google Maps) Category:Earthquakes Category:Seismology Category:Geological hazards Category:Earthquake engineering af:Aardbewing ar:زلزال an:Tierratremo az:Zəlzələ bn:ভূমিকম্প zh-min-nan:Tē-tāng be:Землетрасенне be-x-old:Землятрус bs:Potres br:Kren-douar bg:Земетресение ca:Terratrèmol cv:Çĕр чĕтренĕвĕ cs:Zemětřesení cy:Daeargryn da:Jordskælv de:Erdbeben et:Maavärin el:Σεισμός es:Terremoto eo:Tertremo eu:Lurrikara fa:زمین‌لرزه fo:Jarðskjálvti fr:Tremblement de terre fy:Ierdskodding gan:地震 gd:Crith-thalmhainn gl:Terremoto gu:ધરતીકંપ ko:지진 hy:Երկրաշարժ hi:भूकंप hr:Potres io:Ter-tremo id:Gempa bumi iu:ᓴᔪᑉᐱᓛᕗᖅ/sajuppilaavuq is:Jarðskjálfti it:Terremoto he:רעידת אדמה kn:ಭೂಕಂಪ ka:მიწისძვრა sw:Tetemeko la ardhi la:Terrae motus lv:Zemestrīce lb:Äerdbiewen lt:Žemės drebėjimas hu:Földrengés mk:Земјотрес ml:ഭൂകമ്പം mr:भूकंप ms:Gempa bumi mn:Газар хөдлөлт nah:Tlālollīn nl:Aardbeving ja:地震 no:Jordskjelv nn:Jordskjelv oc:Tèrratrem om:Chocho'a lafa uz:Zilzila nds:Eerdbeven pl:Trzęsienie ziemi pt:Sismo ksh:Äädbevve ro:Cutremur rm:Terratrembel qu:Pacha kuyuy ru:Землетрясение sq:Tërmeti scn:Tirrimotu simple:Earthquake sk:Zemetrasenie sl:Potres szl:Třyńśyńy źymje sr:Земљотрес sh:Potres su:Lini fi:Maanjäristys sv:Jordbävning tl:Lindol ta:நிலநடுக்கம் te:భూకంపం th:แผ่นดินไหว vi:Động đất tg:Заминларза tr:Deprem uk:Землетрус ur:زلزلہ vec:Teremoto wa:Tronnmint d' tere yi:ערדציטערניש zh-yue:地震 zh:地震